In general, there is known an image forming apparatus of an electrophotographic type, in which a transfer bias (transfer voltage) having a polarity different from a charged polarity of developers is applied to a transfer roller arranged to face a photosensitive member, thereby transferring a developer image formed on the photosensitive member to a recording sheet positioned between the photosensitive member and the transfer roller. In the known image forming apparatus, as a resistance value of the sheet is lowered under high-humidity environments, more current than usual flows between the photosensitive roller and the transfer roller, so that the charges of the developers on the photosensitive member are neutralized. Regarding this, one related-art image forming apparatus has been known which under high-humidity environments, performs a printing operation with the transfer bias being reduced, thereby preventing much current from flowing between the photosensitive member and the transfer roller.
In the meantime, another related-art image forming apparatus of a cleanerless type has been known in which the developers remaining on the photosensitive member without being transferred to the recording sheet are collected by a developing roller and the collected developers are reused. In the other related-art image forming apparatus, the developers remaining on the photosensitive member are collected by the developing roller by using a difference between a surface potential of the photosensitive member and a developing bias (developing voltage) applied to the developing roller.